This invention relates to an improved zeolite encapsulating material.
Zeolite is one of adsorbents that are at present widely in use for various purposes including industrial purposes. The zeolite, particularly A type zeolite, is typically represented by a sodium-A zeolite, which is expressed by a typical unit cell of Na.sub.12 (AlO.sub.2.SiO.sub.2).sub.12 .multidot.(NaAlO.sub.2).delta..multidot..omega.H.sub.2 O, wherein 0.ltoreq..delta..ltoreq.1 and .omega. represents a positive number. In this unit cell, 12 sodium ions are ion exchangeable for other metal ions. The effective adsorption pore diameter is determined by the kinds of the exchanged ions and the rate of exchange thereof. However, the size of the effective adsorption pore diameter is in close relation to the crystal structure, the size of the ion species to be exchanged and the site selectivity properties thereof within a unit cell. In other words, among the 12 cations (sodium ions) which are exchangeable within the unit cell of zeolite, three ions are located on the face of an eight-membered oxygen ring where the molecule to be adsorbed comes in and goes out and eight ions are on the face of a six-membered oxygen ring while the remaining one is located on the face of a four-membered oxygen ring.
Therefore, it is the size of the cations on the face of the eight-membered oxygen ring that have an influence directly on the adsorption properties of the zeolite. Where a sodium-A zeolite is employed as starting material and the sodium ions of the zeolite are exchanged for potassium ions, the potassium ions have a preference for entering the sites on the face of the eight-membered oxygen ring. The effective adsorption pore diameter of the sodium-A zeolite is 4 .ANG.. When the potassium ions which are larger than sodium ions enter the sites, the effective adsorption pore diameter of the ion-exchanged zeolite becomes 3 .ANG..
If the ion exchange is carried out for calcium ions, calcium ions have a preference for entering the sites on the face of the six-membered oxygen ring while, among the sodium ions that are to move out to keep the balance of charges, the sodium ions on the face of the eight-membered oxygen ring have priority over other sodium ions in moving out. Therefore, when an ion-exchange process is carried out until all of the sodium ions disappear from the face of the eight-membered oxygen ring, the effective adsorption pore diameter of the zeolite used in the ion-exchange increases and becomes 5 .ANG..
Further, if the sodium-A zeolite is ion-exchanged for a cesium ion which is larger than the potassium ion, the cesium ion has a preference to take a site on the face of the eight-membered oxygen ring. Therefore, the effective adsorption pore diameter becomes smaller than 3 .ANG. when three or more than three cesium ions are exchanged per unit cell.
Generally, the effective adsorption pore diameter of zeolite or that of zeolite obtained through ion-exchange is nearly uniform. A molecule smaller than the effective adsorption pore diameter of the zeolite can be adsorbed by the zeolite. However, a molecule larger than that cannot be adsorbed by the zeolite through a normal process.
The site selectivity properties of the ion to be exchanged with the exchangeable ion contained in the zeolite and variation that takes place in adsorption properties with variation in combination of the species of ions have not been clearly known. The present inventors conducted researches into the details of these relations. As a result of these researches, it has been discovered that a zeolite having a novel adsorption property which has hitherto been unknown can be obtained through an ion exchange process for reformation of zeolite in terms of the adsorption properties thereof carried out with the combination of ion species and the rate of exchange suitably selected. In other words, it has been discovered that, in an ion-exchange process where exchangeable sodium ions of a sodium-A zeolite are gradually exchanged for calcium ions, when two or more than two calcium ions enter, sodium ions on the face of the eight-membered oxygen ring move out and this makes the effective adsorption pore diameter 5 .ANG..
On the other hand, in case where the potassium ions of a potassium-A zeolite, which is obtained by exchanging the exchangeable sodium ions of a sodium-A zeolite with potassium ions, or those of a potassium-A zeolite synthesized with a source of potassium used as raw material, the divalent metal ions have preference to come onto a six-membered oxygen ring face. However, when the number of the divalent metal ions is less than 4.5 per unit cell, the potassium ions on the face of the eight-membered oxygen ring are not removed from there and thus the effective adsorbing pore diameter is kept at 3 .ANG.. Meanwhile, when three or more than three exchangeable sodium ions of a sodium-A zeolite are exchanged for cesium ions and the remainder of sodium ions are exchanged for divalent metal ions, the divalent metal ions have preference to come onto the face of the six-membered oxygen ring and then, with the number of the divalent metal ions less than 4.5 per unit cell, the effective adsorption pore diameter is kept as a value less than 3 .ANG. as determined by the radii of the cesium ions because the cesium ions then stay on the face of the eight-membered oxygen ring.
It has been found that, in the case of a zeolite in which the effective adsorption pore diameter is less than 3 .ANG. with cesium ions on the face of the eight-membered oxygen ring and divalent metal ions on a part of the six-membered oxygen ring face as stated in the foregoing, a molecule of diameter larger than the effective adsorption pore diameter can be adsorbed by the zeolite at a relatively low temperature and at low pressure; and that the adsorbed molecule will not be desorbed even when the zeolite is brought back into ordinary desorbing condition. Namely, the zeolite has an encapsulating properties. This indicates that the cesium ions on the face of the eight-membered oxygen ring are made to be readily movable by the influence of the divalent ions received in exchange. Such a movable state of the cesium ions on the face of the eight-membered oxygen ring is believed to be dependent upon the number of the exchanged divalent metal ions on the face of the six-membered oxygen ring as well as temperature. Therefore, the molecule which is encapsulated in the zeolite can be released from an encapsulated state by raising the temperature of the zeolite. It is also possible that the amount of encapsulated gas and encapsulating and deencapsulating temperature can be controlled by varying the number of the divalent metal ions to be exchanged. In a practical application, the exchangeable cations of the zeolite do not have to be limited to potassium or cesium and divalent metal ions but it is also permissible to have sodium ions on a part of the face of the six-membered oxygen ring.
It has been disclosed by a report appeared in "Journal of the American Chemical Society," Vol. 99, 7074, (1977), Dan Fraenkel and Joseph Shabtai that a cesium-sodium-A zeolite in which exchangeable cations include only cesium and sodium ions is capable of encapsulating hydrogen gas therein.
However, compared with the cesium-sodium-A zeolite, the encapsulating material obtained in accordance with the present invention has greater encapsulating properties and more readily permits encapsulation and deencapsulation. The superiority of the encapsulating material of the invention derives from the fact that the temperature dependency of the thermal vibration of the cesium ions on the face of the eight-membered oxygen ring is increased by the influence of the divalent metal ions received in exchange.
The amplitude of the thermal vibration of the cesium ions on the face of the eight-membered oxygen ring depends not only on the number of the divalent metal ions on the face of the six-membered oxygen ring but also on the temperature condition. Therefore, the molecule encapsulated in the A type zeolite can be deencapsulated by raising the temperature of the zeolite encapsulating the molecule. The radius of the cesium ions is relatively large. Therefore, the gas encapsulating capacity can be increased by minimizing the number of the cesium ions allowed to be on the face of the six-membered oxygen ring besides the three on the face of the eight-membered oxygen ring.